Conventionally, paperboard is coated using a spreading roll or film transfer roll coater combined with air knife doctoring. The base board often has a rough surface as well as inferior and uneven brightness owing to the lesser bleaching of paperboard base stock as compared to bleaching of printing paper. Also brown paperboard is frequently used as the base sheet. The coat layer applied by means of an air knife coater accurately conforms to the surface profile of the base sheet and is of homogeneous thickness, which assures a high opacifying power of the coat layer. Due to such a constant-thickness coat layer, dark-colored or uneven surface of the paperboard base sheet will become efficiently coated, but the surface of the applied coat will not reach the smoothness of a sheet coated with a blade or rod coater. Further, the use of an air knife coater sets a severe limitation to the maximum web speed owing to the weak doctoring effect of the air knife. The solids content of the coating mix must be kept low to prevent premature drying of the coating mix before it reaches the air knife. After the coat is applied to the web, the moisture of the coating mix begins to absorb into the base sheet, whereby the solids content at the web surface begins to increase. Resultingly, the applied coat undergoes formation of two sublayers: a dry lower layer of high solids content on the base sheet surface and, overlying this layer, an upper layer having a solids content approximately equal to that of the virgin coating mix applied on the sheet. The lower sublayer facing the base sheet is called a set coat layer. In practice, the air doctor is only capable of stripping off excess coat down to the interface between the upper and the set sublayers, whereby the final coat thickness can be controlled in air knife coating by adjusting the dwell time between the coat metering step and the doctoring step.
If the coat is doctored by means of an air knife, the maximum solids content of the coating mix to be applied may not exceed 45-53%, because at higher solids content levels the air knife will become inefficient in removing the excess coat from the sheet surface. As the formation of a set coat layer requires a sufficiently long dwell time between the coat metering and doctoring steps, air knife coaters must have the coat applicator and the air knife arranged to operate against separate backing rolls. Another reason for the need of separate backing rolls arises from the fact that the applicator roll and the coat mist collecting chamber of the air knife cannot be accommodated physically about a single backing roll. Typically, the distance between the applicator roll and the air knife is in the order of 1-3 m, whereby the dwell or setting time will be 120-3600 ms for web speeds in the range 50-500 m/min. The longer the dwell time is made, the lower the solids content of the applied coating mix must be in order to achieve a prescribed even coating thickness.
Besides in air knife applicators, paperboard is coated in blade coater stations having the applicator roll adapted to operate against the same backing roll with the doctor blade. A coat smoothed by means of a doctor blade has an even surface, but the thickness of the applied coat layer will be determined by the surface roughness variations of the paperboard base sheet. If the base stock is excessively rough, the risk arises that the doctor blade begins to run supported by the peaks of the roughness profile, whereby such peaks will remain lacking a sufficient amount of coat. As a further result of such a coat of uneven coat layer thickness, the opacifying power of the coat will vary over the sheet thus causing mottled strike-through of the base sheet color. In practice, this problem will be evident for insufficiently bleached paperboard base stock grades only.
The maximum web speed of blade coaters is typically higher than that of air knife coaters, and the speed is chiefly limited by the maximum controllable running speed of the applicator roll. The running speed of the applicator roll is maximally 1500 m/min, and for paperboards, the practical running speed is in the range 300-800 m/min which in a typical coater with a single backing roll designed for a dwell length of 300-600 mm gives a dwell time in the range 22.5-120 ms. As compared to air knife coating, blade doctoring allows a higher solids content of the coating mix, whereby 55-70% solids may typically be used. When an applicator roll is used for spreading the coating layer, the coating mix is pressed into the paper board in the nip of the applicator roll and a backing roll, whereby water is filtered into the base board web and the board roughens due to swelling of the fibers of the board. Also the pigment particles of the coating mix may be pressed deep in the pores of the base web. Because of the preroughening of the base web and the pressing of the pigment particles into the web, the coating mix can not be made to settle evenly on the base web, but the coating layer evens out the unevenness of the surface of the base web while the thickess of the coating layer is uneven. Usually it has been considered that increased dwell times do not give any quality edge in the blade coating.